Question

Why are decomposition reactions called the opposite of combination reactions. Write an equation for these reactions.

Answer 1

Hint: Decompose means break into parts i.e. $A+B\longrightarrow AB$ and combine means adding two or things like $A_{ 2 }+B_{ 2 }\longrightarrow 2AB$ and during decomposition reaction energy is required to breakdown on the other hand, in combination reaction when two or more substances are combined then, energy is released. Hence decomposition reaction can be called as an endothermic reaction while combination reaction is an exothermic reaction.

Complete step by step solution:
 Combination reactions are those reactions in which two or more than two reactants combine to form a single product is called combination or addition reaction. The formation of new bonds takes place between reactants in these reactions.
Examples 1) combustion of coal

$C+ { O }_{ 2 }\quad \underrightarrow { heat } { \quad CO }_{ 2 }$

2) Combustion of magnesium wire

$2Mg+{ O }_{ 2 }\quad\underrightarrow { heat } \quad 2MgO$

on the other hand, the reactions in which a reactant dissociates to form two or more than two products, are called dissociation reactions or decomposition reactions. In these reactions, the bonds between reactants are broken; formation of small molecules takes place, here, reactants are big molecules of high molecular weight which dissociate to form small molecules of low molecular weight.
Temperature, electric current, light etc are responsible for dissociation reactions of substances. On the basis of them, dissociation reaction is of the following types:

A) Electrolytic decomposition: In this type of dissociation reaction. Electric current is passed in the molten and liquid state of the compound.
Example ${ H }_{ 2 }{ O\quad \underrightarrow { electricity } \quad { H }_{ 2 } }\quad +\quad { O }_{ 2 }$
Different products are obtained at anode and cathode in electrolytic decomposition.

 B) Thermal decomposition: In this type of dissociation reaction, the compound on heating dissociates in small molecules.
Example ${ CaCO }_{ 3\quad }{ \underrightarrow { heating\quad } { CaO } }+C{ O }_{ 2 }$
C) Photolytic decomposition: In this type of dissociation, reaction compound dissociates to small molecules by taking energy from light. Since, in these reactions, the dissociation of compounds takes place due to the presence of light. So, these are called photolytic decomposition.

Example $2AgCl\quad \underrightarrow { sunlight\quad } 2Ag + { Cl }_{ 2 }$
Hence, in decomposition reactions there is a breakdown or separation of a chemical compound into its elements or simpler compounds while in composition reactions two or more elements or compounds combine to form a single compound that is why decomposition reactions are called the opposite of combination reactions.

Note: A double decomposition is a kind of reaction in which two constituent reactants interchange cation and anion to form two new compounds so don’t confuse double decomposition reaction with decomposition.